1. Field of the Invention
The present invention relates to an image forming apparatus that includes a first photosensitive drum, and a second photosensitive drum larger in outer diameter than the first photosensitive drum.
2. Description of the Related Art
As an electrophotographic color image forming apparatus, there is a tandem color image forming apparatus that includes yellow, magenta, cyan, and black photosensitive drums. Concerning such a color image forming apparatus, to suppress positional deviation between color images, there has been a proposal to drive a plurality of photosensitive drums respectively by different motors instead of a single motor (refer to Japanese Patent Application Laid-Open No. 2007-047629). The plurality of photosensitive drums are respectively driven by the different motors, and the motors are individually controlled according to rotational speeds of the photosensitive drums. As a result, a difference in rotational phase among the photosensitive drums can be reduced, positional deviation between the color images can be suppressed, and image quality can be improved.
To lower a replacement frequency of a black photosensitive drum by extending a life of the black photosensitive drum which is frequently used, there has been a proposal to make an outer diameter of the black photosensitive drum larger than that of the color photosensitive drum (refer to Japanese Patent Application Laid-Open No. 2007-047629). By making the outer diameter of the black photosensitive drum larger, the circumference of the photosensitive drum is longer, so a deterioration level of the photosensitive drum is smaller when an image is formed on a recording sheet, and the photosensitive drum has a longer life.
Even when the outer diameter of the black photosensitive drum is made larger than that of the color photosensitive drum, a circumferential speed of the black photosensitive drum must be matched with that of the color photosensitive drum. This is because, in order to transfer a toner image formed on each photosensitive drum onto an intermediate transfer belt in contact with each photosensitive drum, a circumferential speed of each photosensitive drum must be matched with that of the intermediate transfer belt. An angular speed of the black photosensitive drum is accordingly lower than that of the color photosensitive drum. Driving torque of the black photosensitive drum is higher than that of the color photosensitive drum.
Normally, when the plurality of photosensitive drums are driven by the different motors, it is sufficient if each driving control is independent, and thus any types of motors can be used. For example, a direct-current (DC) brushless motor maybe used for driving all the photosensitive drums. However, in the case of the DC brushless motor, an angle between magnetic poles is not small, and hence rotation unevenness disadvantageously occurs in a low-speed area (of operation). Thus when the black photosensitive drum of the large outer diameter is driven by the DC brushless motor, rotation unevenness may cause reduction of image quality.
In contrast, a stepping (stepper) motor may be used for driving all the photosensitive drums. However, the stepping motor shows a torque shortage in a high-speed area (of operation), and has a disadvantage of vibrations caused by step-driving. Thus when the color photosensitive drum of a small outer diameter is driven by the stepping motor, countermeasures must be taken against a torque shortage and vibrations.